Randomness and time
UUID v4 uses random bits with fixed version and variant fields. UUID v7 includes a millisecond timestamp and a random part. ULID also combines time and randomness but uses a different, shorter text representation. This tool does not generate UUID v1.
Choosing for a test database
For a conventional random UUID, start with v4. For approximate creation-time ordering, consider v7 and your database's support. This implementation does not promise strict ordering for values generated in the same millisecond. Randomness reduces collision probability but does not replace a unique database index.
Representation and secrecy
Braces, case and removing hyphens change representation, not the identifier's purpose. Check what the receiving system expects. The time fields in v7 and ULID expose approximate creation time. Possession of an identifier should not be the only authorization check for accessing someone else's data.
Questions and answers
Is UUID v1 supported?
No. Available formats are v4, v7 and ULID.
Are collisions impossible?
There is no absolute guarantee. Use a uniqueness constraint in your database.
